Slow down incremental marking on main thread when idle notification is active.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 09a067899813ce395bbe9350ea4241012d29dc33..9fa9497fb5ba9ac156b796e037ed64c19071b6f3 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1295,6 +1295,8 @@ class Heap {
 
   int gc_count() const { return gc_count_; }
 
+  bool RecentIdleNotifcationHappened();
+
   // Completely clear the Instanceof cache (to stop it keeping objects alive
   // around a GC).
   inline void CompletelyClearInstanceofCache();
@@ -2057,6 +2059,9 @@ class Heap {
   // Cumulative GC time spent in sweeping
   double sweeping_time_;
 
+  // Last time an idle notification happened
+  double last_idle_notification_time_;
+
   MarkCompactCollector mark_compact_collector_;
 
   StoreBuffer store_buffer_;